The purpose of the present study is to better define the roles of lymphoid cell subtypes in tumor immunity and to determine which cellular interactions lead most directly to tumor destruction. Procedures will be adapted or developed to increase the populations of cells that interact with the tumor in a detrimental fashion and to decrease the populations of cells that interfere with tumor destruction or in any way protect the tumor. These include cells which are directly inhibitory or which inhibit indirectly by producing suppressor factors. Preliminary work has shown that spleen cells from tumor-bearing mice release a soluble factor which causes immunosuppression. The suppressor factor is non-antigen specific and is produced by T-cells. A primary goal is to further characterize the chemical and biological properties of this factor.